CA2676911A1

CA2676911A1 - Inductive power and data transmission system based on class d and amplitude shift keying

Info

Publication number: CA2676911A1
Application number: CA002676911A
Authority: CA
Inventors: Clemens M. Zierhofer
Original assignee: Individual
Current assignee: MED EL Elektromedizinische Geraete GmbH
Priority date: 2007-02-15
Filing date: 2008-02-15
Publication date: 2008-08-21
Anticipated expiration: 2028-02-15
Also published as: AU2008216116A1; US8023586B2; KR101424842B1; US20080198947A1; RU2009134320A; EP2111692A2; WO2008101151A3; JP5183645B2; EP2111692B1; PL2111692T3; AU2008216116B2; RU2477156C2; CN101636927B; CA2676911C; WO2008101151A2; KR20090112701A; JP2010519818A; CN101636927A

Abstract

A rf signal transfer link is described which uses amplitude shift keying (ASK) to transfer rf data pulses. The link minimizes state transition time a t the end of each data pulse.

Description

Inductive Power And Data Transmission System Based On Class D And Amplitude Shift Keying [0001] This application claims priority from U.S. Patent Application 11/675,176, filed February 15, 2007, the contents of which are hereby incorporated by reference.

Field of the Invention [0002] The present invention relates to signal processing, and specifically, to providing data power signals to implanted systems.

Background Art [0003] One way to provide power and data to an implanted electronic system such as a prosthetic stimulator is to transmit an RF signal via an inductive link. An inductive link basically has two resonant circuits: an external one and an internal one implanted in the patient user. The inductances of the two resonant circuits are realized, for example, as two spiral-shaped coils with typical outer diameters between 20 and 30 mm. When facing each other, the coils form a transformer which allows the transfer of RF-energy.
Inductive links have been investigated with respect to optimizing power transfer efficiency and coupling misalignment tolerance. See, e.g., Galbraith DC, Soma M, and White RL, A Wide-Band Efficient Inductive Transdermal Power And Data Link With Coupling Insensitive Gain, IEEE Trans. Biomed. Eng. BME-34, pp. 265-275, Apr. 1987; and Zierhofer CM and Hochmair ES, High-Efficiency Coupling-Insensitive Power And Data Transmission Via An Inductive Link, IEEE-Trans. Biomed. Eng. BME-37, pp. 716-723, July 1990;
which are incorporated herein by reference.

[0004] In many applications, parallel-tuned receiver circuits are used because the RF-voltage across the resonant circuit can easily be converted to a dc voltage by rectification and smoothing. The dc voltage then is used as a power supply voltage for the electronic circuits within the implanted system. For example, Fig. 1 shows a parallel-tuned receiver resonant circuit of coi1101 and capacitor 102 where signal u2(t) is the induced RF-voltage.
Rectifier diodes 103 and 104 in combination with filtering capacitors 105 and 106 convert the ac voltage u2(t) to a dc-like voltage Vd,. If the filtering capacitors 105 and 106 are sufficiently large, any ac components of Vd, can be neglected. Voltage Vd, is connected to voltage supply ports Vcc and Vss of a subsequent electronic circuit 107 which implements the functionality of the implanted system, e.g., an implanted prosthetic stimulator.

[0005] Signal u2(t) is not only used as supply voltage generation for power, but it also contains digital information data. For example, for a cochlear implant, signal u2(t) provides information defining short biphasic pulses for the electrical stimulation of the acoustic nerve. In general, a bit decoding stage 108 is part of an implanted system that converts the RF-signal u2(t) to a base band bit sequence used for further processing.

[0006] For digital data transfer, at least two different distinguishable states of u2(t) are defined. For example, these two different states could be two different operating frequencies of u2(t), which are in the vicinity of the resonance frequency f2.
Such a scheme is usually designated as Frequency-Shift-Keying (FSK). A practical example is described, e.g., in Galbraith above, where f2 = 20 MHz, and the two operating frequencies are 19 MHz and 21 MHz.

[0007] Another way to encode digital information in signal u2(t) is with Amplitude Shift Keying (ASK). In an ASK-scheme, the two distinguishable states of u2(t) can qualitatively be described by "RF-amplitude present" and "no RF-amplitude present". These two (ideal) states can easily be detected by means of envelope detection. For example, in Fig. 1 decoding stage 108 would then include an envelope detector.

[0008] In Fig. 2, an equivalent circuit of an inductive link system is shown.
The parallel-tuned receiver circuit includes receiver coi1201, capacitor 202, and resistor 208, where resistor 208 represents the ohmic losses due to the parasitic resistance of coi1201.
Resonance frequency f2 and unloaded quality factor Q2,,,,,ioadea are defined as f 2 27r L2C2 , (1) and, FL . (2) Q2,unloaded = R2 Th e power consumption of stage 107 in Fig. 1 is represented by an ohmic load 207.
Rectifier diodes 103 and 104 are represented by simple equivalent circuits 203 and 204, which themselves are composed of ideal switches 2031 and 2041, and ohmic resistors 2032 and 2042. The states of the switches depend on voltage u2(t) and voltages VA and VB
across capacitors 205 and 206, respectively. It is assumed that switch 2031 is closed if u2(t) > VA, and it is in its high impedance state for u2(t) <_ VA. Similarly, switch 2041 is closed if u2(t) < -VB, and opened for u2(t) _ -VB.

[0009] Receiver coi1201 is inductively coupled to a transmitter coi1209, and the coupling strength is described by coupling coefficient k. Transmitter coi1209 together with capacitor 210 and resistor 211 form a series-tuned transmitter resonance circuit, where resistor 211 represents the parasitic resistance of coi1209. Resonance frequency fi and unloaded quality factor Qi,,,nioaded are defined as:

f, =
2~ LiCi , (3) and, rcii (4) Ql,unloaded - R

The input of the transmitter circuit is driven by voltage source 212 which generates input voltage ui(t). For ASK, typically two modes of operation, i.e., states RF-ON
and RF-OFF, are used. As depicted in Fig. 3, in state RF-ON, ui(t) is switched periodically between ground potential and a supply voltage VDD. Period T denotes the RF-period.
During state RF-OFF, ui(t) is connected to ground potential.

[0010] Figure 4 shows an example of voltage ui(t) for a sequence of bits using a self clocking bit format. Here, a logical "0" is encoded into a sequence RF-ON
followed by RF-OFF, and vice versa, a logical "l" is encoded into a sequence RF-OFF
followed by RF-ON.

Summary of the Invention [0011] Embodiments of the present invention are directed to an rf signal transfer link which uses amplitude shift keying (ASK) to transfer rf data pulses. The link includes means for minimizing state transition times.

[0012] For example, the means for minimizing may include means for changing a resonant circuit quality factor as would be useful in a parallel-tuned receiver circuit for receiving the rf data pulses or a series-tuned resonant transmitting circuit for transmitting the rf data pulses. For example, a series-tuned resonant transmitting circuit includes a class D
amplifier driver. In a more specific embodiment, the class D amplifier may be integrated onto a single microchip. The series-tuned resonant transmitting circuit may include a damping resistor RD in series with a transmitter circuit inductance during the rf off time.
For example, the damping resistor RD may be an open circuit with infinite resistance, or it may have an optimal resistance to minimize the state transition times. In some embodiments, the damping resistor RD may cause the resonant receiving circuit to behave such that rf waveform decay is independent of coupling factor.

[0013] Embodiments also include a receiver circuit for an implanted electronic system. An implanted receiver circuit receives amplitude shift keyed (ASK) rf data pulses from an external transmitter. The receiver circuit includes means for minimizing state transition times. In some embodiments, the means for minimizing may include means for changing a resonant circuit quality factor.

[0014] Embodiments also include a transmitter circuit for an implanted electronic system.
An external transmitter circuit transmits amplitude shift keyed (ASK) rf data pulses to an implanted receiver. The transmitter circuit includes means for minimizing state transition times. The means for minimizing may include means for changing a resonant circuit quality factor, such as a series-tuned resonant transmitting circuit for transmitting the rf data pulses. The series-tuned resonant transmitting circuit may include a class D amplifier driver. In a more specific embodiment, the class D amplifier may be integrated onto a single microchip. The series-tuned resonant transmitting circuit may include a damping resistor RD in series with a transmitter circuit inductance during the rf off time. For example, the damping resistor RD may be an open circuit with infinite resistance, or it may have an optimal resistance to minimize receiving circuit state transition times. In some embodiments, the damping resistor RD may cause the resonant receiving circuit to behave such that rf waveform decay is independent of coupling factor.

[0015] Embodiments of the present invention also include an rf transfer link which uses amplitude shift keying (ASK) to transfer rf data pulses. The link includes means for causing the system to behave such that rf waveform decay is independent of coupling factor.

Brief Description of the Drawings [0016] Figure 1 shows a parallel tuned receiver resonant circuit and power supply voltage generation according to the prior art.

[0017] Figure 2 shows an equivalent circuit of an inductive link with series-tuned transmitter and parallel-tuned receiver resonant circuit according to the prior art.

[0018] Figure 3 shows modes of RF-ON and RF-OFF of input voltage ui(t) for Amplitude Shift Keying (ASK).

[0019] Figure 4 shows an example of ui(t) for a bit sequence using a self clocking bit format.

[0020] Figure 5 shows an equivalent circuit of an inductive link with series-tuned transmitter and parallel-tuned receiver resonant circuit according to one embodiment of the present invention.

[0021] Figure 6A-C shows state signals and voltage traces for various circuit values in a receiver resonant circuit.

Detailed Description of Specific Embodiments [0022] An ASK-based signal transfer link system for data and energy transmission as in Fig. 2 has a potential problem. Assuming a periodic sequence of input states RF-ON and RF-OFF of voltage ui(t), and also assuming steady state conditions, dc-voltage Vd, across load Rd, is constant. During state RF-ON, voltage u2(t) reaches a peak amplitude which is slightly higher than Vd,/2. Switches 2031 or 2041 are closed if u2(t) > Vd,/2, or if u2(t) <-Vd,/2 respectively. Only during these very short periods charge is flowing into the network CA, CB, Rd,. However, these diode currents define a particular loaded quality factor Q2,1oadea, which is considerably smaller than the unloaded quality factor Q2,unloaded as defined in Equation (2) above.

[0023] If ui(t) changes from state RF-ON to RF-OFF, the voltage amplitude of u2(t) cannot immediately follow such a change. Instead, it will take some time for the RF
amplitude to decrease back towards baseline, and the velocity of decay is strongly influenced by the quality factor of the receiver circuit: the lower the quality factor, the faster the decay is. Unfortunately, basically the unloaded quality factor Q2,.ioadea which is much higher than the loaded quality factor Q2,1oadea applies here because as soon as the amplitude of u2(t) falls below Vd,/2, diode switches 2031 and 2041 remain open, and no RF energy can flow into the network CA, CB, Rd,. During RF-OFF, the only effective ac-load within the receiver circuit is R2.

[0024] The exact relaxation behavior of u2(t) during RF-OFF is determined by the network Ri, Ci, Li, R2, C2, L2, and coupling factor k, and thus the unloaded quality factors Qi,,,nioadea and Q2,unloaded are relevant. However, these quality factors should be as high as possible with respect to a high power transfer efficiency during RF-ON. So the requirements for high power efficiency and fast RF-relaxation during RF-OFF in an ASK
scheme are contradicting each other.

[0025] One way to address this problem is to decrease the quality factor in the transmitter resonant circuit during RF-OFF so that each rf data pulse will end with a more rapid decrease in pulse amplitude. One specific embodiment of a signal transfer link is shown in Fig. 5, which is a system for amplitude shift keying (ASK) transfer of rf data pulses. Block 501 represents the inductive signal transfer link. Block 501 includes an external series-tuned resonant transmitter circuit for transmitting the rf data pulses using ASK, and an implanted parallel-tuned receiver module for receiving the rf data pulses. The input node 502 is connected to a network composed of a switching pair 503 and 504, resistor 505 and switch 506. During state RF-ON, switch 506 is open (high impedance), and node 502 is switched between ground potential and supply voltage VDDby means of switching pair 503 and 504. This causes a rectangular voltage at radio frequency, and such an operating mode is usually designated as a class D switching paradigm. During state RF-OFF, switches 503 and 504 are open, and switch 506 is closed. Now RDseries damping resistor 505 is connected to Ri in series, which means a considerable reduction of the transmitter quality factor. This reduction of the quality factor for the transmitter resonant circuit at the end of each rf data pulse accelerates the decay of the RF amplitude in the receiver resonant circuit so as to minimize the time required for the trailing edge of the pulse to rapidly decrease back to baseline.

[0026] A quantitative analysis for the circuit in Fig. 5 has been carried out with specifications as summarized in Table 1. The simulations are based on a state space model with a computation time increment of 0.5 ns. The RF frequency is 10 MHz, resulting in T
= 100 ns. Simulation results are shown in Fig. 6A-C where the upper traces depict signal state. For STATE = HIGH, the class D driver produces an RF-signal (state RF-ON), and for STATE = LOW, the input of the transmitter series circuit is connected to RD(state RF-OFF). Here, a self-clocking bit format with bit duration of 2 s is assumed.
Note that the shortest possible duration for RF-ON is 1 s, which contains exactly 10 RF
cycles.

Table 1: Circuit Specifications k 0.2 Li 2.70 H
L2 0.85 H
Ci 82 pF
C2 260 pF
CA 10 nF
CB 10 nF
Ri 2 S2 Rdiode 40 S2 Rde 10 kS2, Further simulations have been computed for different values of RD at a coupling of k 0.2. Figure 6A shows the result for RD = 0. Obviously, during periods STATE =
0, the amplitude of u2(t) is decaying quite slowly and the decay is superimposed by a typical beat effect. Energy is oscillating between the transmitter and receiver resonant circuits with a beat frequency of about 2 MHz. Although the data structure can be identified visually, data detection by means of an electronic circuit cannot be easily achieved.

[0027] If the damping resistor is set to RD --> cc as shown in Fig. 6B, the beat effect disappears. In this case, the receiver resonant circuit relaxes as if it was not coupled to the transmitter because the current in the transmitter is forced to zero. Thus the behavior of a 2nd order system is observed, i.e., the amplitude of u2(t) decays exponentially according to the time constant of the unloaded receiver circuit L2, C2, R2. The waveform u2(t) here is less complex as compared to Fig. 6A. Note that the decay for RD -> cc is independent of coupling factor k, which might be an important feature for subsequent data decoding stages. For example, in a cochlear implant system this is an important criterion because the data decoding should be insensitive the mutual coil positions.

[0028] Figure 6C shows the case for RD = 150 Q. Voltage u2(t) clearly outperforms Figs.
6A and 6B, decaying comparatively fast at the end of each rf data pulse. Thus u2(t) is very well suited for data decoding.

[0029] One intrinsic advantage of embodiments such as the one shown in Fig. 5 is that many components of the external system can be integrated onto a single microchip, and thus power consumption and system size can be kept very small. Also, an rf generator (not shown in Fig. 5) to drive switches 503 and 504 during RF-ON can easily be integrated onto such a microchip. The performance of switches 503 and 504 is of particular importance: the ON-resistances typically should not exceed 1 52,, and the gate capacitances should be smaller than 10 pF. However, such characteristics can be achieved with existing technologies such as, e.g., 0.35 m CMOS. In many signal transfer link applications, it may also be important that switches 503 and 504 are not closed simultaneously during RF-ON. Otherwise, very large currents can flow through the switches which dramatically enhance the power consumption and may damage circuit components.

[0030] Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the true scope of the invention.

Claims

1. An rf signal transfer link comprising:
a signal transfer link for using amplitude shift keying (ASK) to transfer rf data pulses, the link including means for minimizing state transition times.

2. A link according to claim 1, wherein the means for minimizing includes means for changing a resonant circuit quality factor.

3. A link according to claim 1, further comprising:
a parallel-tuned receiver circuit for receiving the rf data pulses.

4. A link according to claim 1, further comprising:
a series-tuned resonant transmitting circuit for transmitting the rf data pulses.

5. A link according to claim 4, wherein the series-tuned resonant transmitting circuit includes a class D amplifier driver.

6. A link according to claim 5, wherein the class D amplifier is integrated onto a single microchip.

7. A link according to claim 4, wherein the series-tuned resonant transmitting circuit includes a damping resistor RD in series with a transmitter circuit inductance during the rf off time.

8. A link according to claim 7, wherein the damping resistor RD is an open circuit with infinite resistance.

9. A link according to claim 7, wherein the damping resistor RD has an optimal resistance to minimize receiver circuit state transition times.

10. A link according to claim 7, wherein the damping resistor R D has a resistance which causes the resonant receiving circuit to behave such that rf waveform decay is independent of coupling factor.

11. A receiver circuit for an implanted electronic system, the circuit comprising:
an implanted receiver circuit for receiving amplitude shift keyed (ASK) rf data pulses from an external transmitter, the receiver circuit including means for minimizing state transition times.

12. A receiver circuit according to claim 11, wherein the means for minimizing includes means for changing a resonant circuit quality factor.

13. A transmitter circuit for an implanted electronic system, the circuit comprising:
an external transmitter circuit for transmitting amplitude shift keyed (ASK) rf data pulses to an implanted receiver, the transmitter circuit including means for minimizing state transition times.

14. A transmitter circuit according to claim 13, wherein the means for minimizing includes means for changing a resonant circuit quality factor.

15. A transmitter circuit according to claim 13, further comprising:
a series-tuned resonant transmitting circuit for transmitting the rf data pulses.

16. A transmitter circuit according to claim 15, wherein the series-tuned resonant transmitting circuit includes a class D amplifier driver.

17. A transmitter circuit according to claim 16, wherein the class D amplifier is integrated onto a single microchip.

18. A transmitter circuit according to claim 15, wherein the series-tuned resonant transmitting circuit includes a damping resistor R D in series with a transmitter circuit inductance during the rf off time.

19. A transmitter circuit according to claim 18, wherein the damping resistor R D is an open circuit with infinite resistance.

20. A transmitter circuit according to claim 18, wherein the damping resistor R D has an optimal resistance to minimize receiver circuit state transition times.

21. A transmitter circuit according to claim 18, wherein the damping resistor R D has a resistance which causes the resonant receiving circuit to behave such that rf waveform decay is independent of coupling factor.

22. An rf transfer link comprising:
a signal transfer link for using amplitude shift keying (ASK) to transfer rf data pulses, the link including means for causing the system to behave such that rf waveform decay is independent of coupling factor.